Syrian government forces on Thursday shelled rebel positions in Damascus with mortar fire in what observers have described as the most violent fighting in the capital since the beginning of the month.

The violence coincides with the departure of UN observers, whose mandate expired and not renewed because of the deteriorating security conditions. The head of the UN mission, Babacar Gaye, was to leave Syria on Thursday as some observers have already left Wednesday.

The Syrian artillery came into action from the hills overlooking Damascus in Qasioum, said the rebels. Shootings were heard in the southern districts of the capital. "We heard explosions and gunfire for about an hour. This is not as violent as Wednesday but the tension remains very strong. I think you can hear the echo of bombings and clashes in most districts of Damascus, "said an opposition activist.

According to him, the shelling seems concentrated on the southern suburbs of Damascus where insurgents have infiltrated. On Thursday morning, government troops have operated in the district of Kafr Souseh where they made arrests. Hundreds of people have fled the fighting in the city and others have found refuge in mosques and schools in the area.

Meanwhile, the regime forces, backed by helicopters and heavy artillery, launched a major offensive in Aleppo, the second largest city of Syria. According to the Syrian Observatory of Human Rights (SOHR), violence killed at least 115 people on Wednesday.

The SOHR also accused government forces of killing 42 civilians, some of whom were summarily executed Tuesday in Maadamiyat al-Sham, 6 km from Damascus.

The Syrian National Council (SNC), the main opposition coalition, for its part, denounced a "brutal crime" in Maadamiyat al-Sham, a village "devastated" by regime forces and where people are in urgent need of medical and humanitarian aid.

In Aleppo, the air force bombed rebel areas south of the metropolis, where insurgents fiercely resisted the troops for over a month. The army shelled neighborhoods east of the city and fighting raged in several other districts, according to the SOHR.